Photolithographic techniques may be used in the fabrication of semiconductor devices to transfer a pattern (e.g., a circuitry pattern) to a wafer using light or other wavelengths of electromagnetic radiation. The “light” may pass through, or be reflected off of, a mask which defines the pattern. Light from the mask projects an image of the pattern onto the wafer. The wafer is coated with a layer of a photosensitive material, referred to as a “photoresist” or “resist,” which undergoes a chemical reaction when exposed to light. After exposure, the resist is baked and developed, leaving regions of the wafer surface covered resist and complementary regions exposed.
The resolution and efficiency of photolithographic systems may be affected by the amount of light coupled into the photoresist. The optical absorbance of photoresist materials used in lithography has increased with decreasing wavelength, especially in the Extreme Ultra-Violet, or EUV (˜13.5 nm). As a result less light reaches the underlying substrate (˜50% in the EUV). A decreasing index of refraction mismatch between the photoresist and the underlying substrate has also reduced reflections from the substrate back into the photoresist. The net effect is wasted photons.